<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input {
            display: none;
        }

        input:checked+label {
            color: red;
        }

        .hide {
            width: 200px;
            height: 300px;
            background-color: #f00;

            height: 0;
            overflow: hidden;
            transition: .3s;
        }

        input:checked~.hide {
            height: 300px;
        }
    </style>
</head>

<body>

    <ul>
        <li>
            <input type="checkbox" id="a1">
            <label for="a1">教学系统</label>
            <div class="hide">
                <p>111</p>
                <P>22</P>
            </div>
        </li>
        <li>
            <input type="checkbox" id="a2">
            <label for="a2">我的学习</label>
            <div class="hide"></div>
        </li>
    </ul>

</body>

</html>